Why Salesforce Predefined Values for an Email Quick Action Are Not Used

Predefined values set on an email action for the To, CC, or BCC fields will only work as a default for sending an email for the first time or when there is no existing emails thread for the record (no emails were previously sent from or received on the record).

The email composer remembers the email thread along with the values previously used on the last email.  If there is already an existing email thread for the record you are trying to send emails from, then the email composer will no longer use the predefined value and will instead use the values previously used in the email.

The email thread is shared across all email actions, hence even if you use the newly created email action, there will be an email thread in it.

The From field is always populated with the predefined value unless QuickActionDefaultsHandler is present. See Understand How Default Values for Case Emails Work for more details.

  
Note: When Email Drafts are enabled, pre-defined field values are not respected when they are not present on the email quick action layout.